Manganese activated zinc silicate (commercially referred as P1) is a very efficient green emitting phosphor second only to copper activated zinc sulfide. P1 has been extensively used in many cathode ray tube displays. When arsenic is added to the Mn activated zinc silicate, the phosphor (P39) exhibits long persistence (150 millisec. at 10% point) in cathodoluminescence. Although the latter phosphor has useful applications in storage cathode-ray-tubes (CRT) and computer displays, there is very little prior art revealing the exact role of As in the silicate system.